Direct Diagnosis by DNA Analysis of the Fragile X Syndrome of Mental Retardation

Abstract

Direct diagnosis by DNA analysis is now an efficient and reliable primary test for the diagnosis of the fragile X syndrome after birth, as well as for prenatal diagnosis and genetic counseling.

Hotspot for deletions in the CGG repeat region of FMR1 in fragile X patients

“…Reduction of a premutation to a smaller premutation has been detected (6.7,15-17) as well as reduction of a premutation to a repeat length within the normal range (18). Decrease from a full mutation to a premutation has been reported only once (15), whereas the reverse mutation, from full mutation to a normal allele has not yet been described. This is in contrast to the CTG repeat in myotonic dystrophy, in which a number of independent reverse mutations have already been described (19)(20)(21).…”

Chromosomal fragility and human genetic disorders

“…The situation is also complicated by recombination involving the fragile X locus. All things considered molecular diagnosis with the use ofa DNA probe mapping at FMR1 gene proved to be the most reliable and direct method in both the carriers of the fragile X syndrome as well as the affected individuals, regardless of their sex (37). It is also reliable and applicable for prenatal diagnosis.…”

“…Using restriction enzymes that are inhibited by DNA methylation it is possible to detect the sequences that are abnormally methylated on the X chromosome in females that can be distinguished from the fragments corresponding to the premutation or the normal sequence which remain unmethylated only on the active X chromosome. Direct diagnosis of the fragile X syndrome by the digestion with restriction endonucleases such as EcoRI, Hindlll, Pstl and Bcll has greatly improved the chances of detection of fragile X syndrome (37). These enzymes can cleave the DNA sequences on either side of the region where CGG repeat is present.…”

“…Therefore the length of the fragment derived from this region of the FMR 1 gene will vary depending upon the number of the CGG repeats present (Fig 4). By Southern hybridization, using probes derived from sequences 3' or 5' to the (CGG) repeat that are part of the larger fragments large deletions involving CGG repeat and flanking sequences as well as the methylation status of the FMR1 in an individual can be detected (37).…”

Molecular genetic screening in cytogenetically normal mentally retarded males with manifestations of fragile X syndrome

“…In order to identify if cytogenetically normal (using two induction systems [folate deficient culture medium and folate replete culture medium but with 5-fluorodeoxyuridine] to enhance the expression of fragile sites) yet mentally retarded males without a known cause of their retardation had expansion of the CGG repeat segment of the FMR-1 gene, Southern blot analysis was performed using two probes (fxa241 [Oncor, Inc., Gaithersburg, MD] and Ox1.9 [kindly provided by Dr. Kay Davies]) following digestion of genomic DNA with PstI and EcoRI/EagI, respectively, using established protocols [Kremer et al, 1991;Rousseau et al, 1991]. DNA studies were performed on 20 (12.3%) of the previously examined 162 (122 whites and 40 blacks; age range 7-77 years with an average age of 40.0 ± 14.4) cytogenetically normal mentally retarded males without a known cause of their retardation but with high anthropometric discriminant values and/or clinical checklist scores as described elsewhere [Butler et al, 1991a, b].…”